Apparatus for the manufacture of phosphorous acid

ABSTRACT

Phosphorous acid is produced from PCl 3  and water in an apparatus, the essential parts of which include a reactor and an evaporator, which are connected together by means of a conduit running from the bottom of the reactor to the head of the evaporator. The evaporator communicates with the lower end of a separator of which the upstream end terminates in an off-gas outlet. More particularly, the downstream end of the evaporator is connected to a stripping column which is provided with an off-gas outlet opening into the off-gas outlet associated with the evaporator.

This is a divisional application of Ser. No. 319,464, filed Dec. 29, 1972, which issued as U.S. Pat. No. 3,859,419.

The present invention relates to a apparatus for the manufacture of phosphorous acid from phosphorus trichloride and water as starting materials, which comprises introducing the finely divided starting materials into a reaction zone; intimately mixing the starting materials and reacting them therein, in gas phase at temperatures between 150° and 200°C; conveying resulting gaseous reaction products to the atmosphere, through a separating zone and preferably through a hydrogen chloride absorption system; delivering resulting liquid reaction product to an evaporation zone and concentrating it therein so as to produce a melt, flowing an inert gas therethrough and removing the melt.

A similar process and apparatus have already been described in German Pat. No. 1 206 406.

This latter process, of which the chemism appears to indicate that PCl₃ is hydrolyzed in gas phase, substantially comprises finely dividing the PCl₃ and H₂ O starting materials so as to enable more rapid reaction and fairly simultaneously mixing the starting materials together and reacting them. The components can be sprayed mechanically in conventional manner through single or two-opening nozzles using air, nitrogen of CO₂ as the spray-inducing means, and mixed together with the use of steam. This is not fully satisfactory, however, as the spraying is effected using additional inert gases as the spray-inducing means. These, however, are ballast materials which affect the reaction and considerably affect the throughput for a given volume of apparatus, i.e. reduce the apparatus' capacity.

We have now unexpectedly discovered that it is possible by appropriately modifying the apparatus and apparatus reported in the above German Patent Specification to improve the yield and quality of phosphorous acid and increase the capacity of the apparatus used for making it. To this end, the present invention provides for the phosphorus trichloride and the water to be sprayed together by means of steam; for an inert gas to be passed through the reaction product in a further separation zone separate from the evaporation zone; and for the gases issuing from the said further separation zone to be delivered to the atmosphere, without passage through the separation zone receiving the gaseous reaction products. The gas coming from said further separation zone should more conveniently be delivered to the atmosphere via the hydrogen chloride absorption system.

The separating zone receiving the gaseous reaction products should preferably be fed with the quantity of water and/or phosphorous acid, which is necessary to maintain a maximum temperature between 150° and 200°C, preferably 160° and 190°C, therein. It has also been found advantageous to inspect the gases issuing from the separation zones as to the formation of smoke and to regulate the supply of starting materials in accordance therewith.

In view of the fact that the steps of mixing the starting materials together and spraying them are both carried out with the use of steam, it is unnecessary to use further inert gases as the spray-inducing means. As a result, the liquid PCl₃ and H₂ O reactants are finely sprayed and at the same time they are particularly rapidly and uniformly heated to the desirable reaction temperature.

Following the start of the reaction and following liberation of the heat of reaction, it is possible for the water with its high evaporation heat to act -- this in a manner analogous to that described in the above earlier process -- as an intensive so-called inner coolant inhibiting temperature rises and decomposition of the H₃ PO₃ produced. We have also discovered that the quality of the final product can be improved by completely dehydrochlorinating and cooling the H₃ PO₃ -melt in a column rather than in the evaporator itself, the column being placed downstream of the evaporator, and by uniting the gas mixture issuing from the column with the off-gases coming from other structural elements forming part of the apparatus, just ahead of the HCl-absorption system. The increase in capacity rendered possible thereby for an apparatus having a given volume is critically determined by the temperature rise in the reflux column, as increasing quantities of PCl₃ are carried along thereinto at increasing flow velocities and undergo belated reaction therein. While high reaction temperatures have been found to promote the formation of highly concentrated H₃ PO₃, the fact remains that the use of temperatures higher than substantially 200°C increasingly effect the decomposition of H₃ PO₃ in the gas phase, inter alia in accordance with the equation 4 H₃ PO₃ → 3 H₃ PO₄ + PH₃. It should not be ignored, however, that both the resulting PO₄ -content in the final product and the PH₃ which appears in the exhaust gas are undesirable.

The introduction of definite proportions of H₂ O and/or dilute H₃ PO₃ into the reflux column has now been found to enable a desirable optimum temperature range between 160° and 190°C to be established therein and the throughput and energetic balance of the system to be further improved. The introduction of these relatively small proportions of liquid matter into the upper portion of the column has also been found completely to move the equilibrium of the gas phase hydrolysis to the H₃ PO₃ side and to thereby increase the yield up to substantially 98 %. Still further, the purity and quality of the final product are considerably improved. As compared with products made by the conventional process, it was possible to increase the H₃ PO₃ -concentration up to 99.5 %, to reduce the PO₄ -content from formerly less than 1.4 % down to less than 0.2 %, and the chloride content from formerly less than 500 ppm (ppm stands for parts per million) down to less than 10 ppm.

In order to have optimum conditions in carrying out the process of the present invention, it is also necessary to take specific steps which are intended to stabilize the reaction and, in the end, to minimize control and maintenance. In this connection, it has been found advantageous to supply the liquid starting materials, particularly the PCl₃ -reactant, with the use of dosing pumps. These are very useful as they enable the apparatus to be fed with constant quantities of material and enable the reactor and reflux column to be set to pulsating operation. Especially in those cases in which the quantity control is achieved mechanically and the supply of liquid feed material is controlled by means of valves, it is avantageous to provide a photo-electric safety device monitoring and inspecting the off-gas for clearness. This is desirable in view of the fact that underfeeding or overfeeding of PCl₃ and/or H₂ O if it ever occurs -- for example as a result of unexpected sudden changes in the cross-sectional area caused by foreign substances -- has been found to entail temperature rises which effect the decomposition of H₃ PO₃ and, in the end, effect strong formation of smoke in the issuing gas. As soon as this is the case, the photo-electric safety device, which responds thereto, and a control element placed downstream thereof, for example a pneumatic valve, cause the supply of PCl₃ to be reduced or stopped.

The process performed by the present invention should preferably be carried out in the exemplary apparatus shown diagrammatically in the accompanying drawing.

As can be seen, a reactor 2 is fitted at its upstream end with a two-opening nozzle 1 supplying PCl₃ and H₂ O and with a steam inlet 3 opening against the orifice relief of nozzle 1, the two nozzle openings being preferably arranged coaxially with respect to one another. The reactor 2 should conveniently be provided with a second steam inlet 4 projecting laterally thereinto and supplying the reactor with steam for mixing the reaction components together.

A branch conduit 6 connected to conduit 5 connecting the downstream end of reactor 2 to an evaporator 12 is arranged to run to a separator 7, which takes the form of a reflux condenser. At its upstream end, separator 7 is provided with a feed pipe 8 supplying water and/or H₃ PO₃ and with an off-gas outlet 9 which runs from separator 7 through a photo-electric safety device 10 to a conventional HCl-absorption system 11 (not shown in the drawing).

A conduit 13 runs from the downstream end of evaporator 12 to a stripping column 14. A H₃ PO₃ -melt is introduced thereinto from above and nitrogen is introduced from below, the nitrogen coming through conduit 15 travelling upwardly in column 14 countercurrently with respect to the melt. The purified and concentrated H₃ PO₃ -melt is removed through the downstream end of column 14 and conveyed through outlet 16 to a crystallizer 17, whilst the gases are removed near the upstream end of column 14 through conduit 18, which is connected to conduit 9 and opens thereinto at a place between separator 7 and photo-electric safety device 10.

The following Examples further illustrate the process of the present invention and its technically beneficial effects.

EXAMPLE 1

A reactor was fed with 36 kg/h of PCl₃ which were sprayed thereinto through a two-opening nozzle using 1.5 cubic meters per hour of N₂ as the spray inducing means. The reactor was fed simultaneously with 70 kg/h of steam. The reaction temperature varied between 170° and 190°C. It was 190°C in the continuous heater. To remove residual H₂ O and HCl, the heater was fed with 3.0 cubic meters/h of N₂. The off-gases coming from the reactor and evaporator were delivered to a separator which took the form of a reflux column.

20 Kg/h of a 97 % H₃ PO₃ were obtained in a yield of 93 %. The H₃ PO₃ contained 0.9 % of PO₄ and 300 ppm of Cl.

EXAMPLE 2

A reactor the same as that used in Example 1 was fed with 56 kg/h of PCl₃ and 18 l/h of H₂ O which were jointly sprayed thereinto and heated by means of 100 kg/h of steam. The temperature varied between 170° and 180°C. It was 210°C in the continuous heater. Residual water and hydrogen chloride were removed and the H₃ PO₃ -melt was cooled in a stripping column which was placed downstream of the evaporator and which was fed with 2.0 cubic meters/h of N₂. The off-gases coming from the stripping column were united with the off-gases coming from the reactor, downstream of the separator.

32 Kg/h of 98 % H₃ PO₃ were obtained in a yield of 96 %. The acid contained 0.5% of PO₄ and 100 ppm of Cl.

EXAMPLE 3

A reactor the same as that used in Examples 1 and 2 was fed with 116 kg/h of PCl₃ and 25 l/h of H₂ O which were jointly sprayed thereinto and heated by means of 120 kg/h of steam. A further 12 l/h of H₂ O or 15 l/h of a 20 % H₃ PO₃ were introduced into the head of the separator, which was a reflux column.

The reaction temperature varied around 185°C. It was 214°C in the continuous heater. To remove residual H₂ O and HCl and to cool the H₃ PO₃ -melt, the stripping column was supplied with 4 cubic meters/h of N₂.

The off-gases coming from the stripping column and the off-gases coming from further parts of the apparatus were united downstream of the separator.

68 kg/h of H₃ PO₃ with a strength of more than 98.5 % were obtained in a yield of 98.0 %. The acid contained 0.2 % of PO₄ and 10 ppm of Cl.

The yields obtained in the above Examples are based on P in the PCl₃ and the percentages are by weight. 

We claim:
 1. In an apparatus for producing phosphorous acid from PCl₃ and water including a reactor (2) having a two-opening nozzle (1) mounted thereon, of which the two openings are preferably arranged coaxially with respect to one another and having an orifice relief opening into the reactor (2) for atomizing the PCl₃ thereinto; a steam inlet (4) opening into the reactor (2) laterally with respect thereto for introducing steam thereinto; a conduit (5) connecting the downstream end of reactor (2) with an evaporator (12) and conveying liquid reaction product coming from reactor (2) to evaporator (12) for converting the liquid product to a melt, therein; a further conduit (6) running from the conduit (5) to a separator (7) and delivering escape gas loaded with unreacted, PCl₃ from evaporator (12) to separator (7) of which the upstream end terminates in an off-gas outlet (9) delivering off-gas to a HCl-absorption system; the improvement according to which nozzle (1) is provided with an H₂ O-inlet permitting PCl₃ and H₂ O to be stream-sprayed jointly into the reactor (2); reactor (2) is provided with an additional steam inlet (3) opening against the orifice relief of nozzle (1) for blowing steam into reactor (2) a stripping column (14) is provided; evaporator (12) is provided with a conduit (13) running from, and connecting, its downstream end to the upstream end of the stripping column (14), for conveying the melt from evaporator (12) to the stripping column (14); the stripping column (14) is provided at its downstream end with a gas inlet (15) opening thereinto for blowing inert gas through the melt therein; and further provided at its downstream end with an effluent outlet (16) for conveying the melt to a crystallizer (17); and connected to off-gas outlet (11) by means of a conduit (18) running from the upstream end of column (14) to off-gas outlet (11) at a place between the separator (7) and a photoelectric safety device (10) monitoring and inspecting the clearness of the off-gas.
 2. The apparatus as claimed in claim 1 wherein separator (7) is provided at its upstream end with a feed pipe (8) for supplying water and/or H₃ PO₃ into separator (7). 